1. Field of the Invention
The present invention relates to an antenna device preferably for use in, for example, HF-band communication, and relates to an electronic apparatus including the antenna device.
2. Description of the Related Art
A planar coil antenna has been used as an antenna device for an RFID (radio frequency identifier) or NFC (near field communication). In general, the planar coil antenna includes a base material and a rectangular spiral conductor pattern provided on the base material.
Japanese Unexamined Patent Application Publication No. 11-3411 discloses an antenna device in which the inductance of such a planar coil antenna can be adjusted and set to a predetermined value. In the antenna device of Japanese Unexamined Patent Application Publication No. 11-3411, a conductor pattern is provided on each surface of a base material, an antenna coil having a plurality of turns is provided on a front surface of the base material, and a one-turn coil partly opened is provided on a back surface of the base material to have almost the same shape as that of a region defined by the outermost periphery and the innermost periphery of the antenna coil.
When an antenna device is included in an electronic apparatus such as a mobile communication terminal, it is required to obtain a predetermined gain with a more compact size along with recent reduction in size and thickness of electronic apparatuses.
In the antenna device disclosed in Japanese Unexamined Patent Application Publication No. 11-3411, an equivalent inductance of the antenna coil is determined within a certain range by the capacitance generated between the antenna coil and the one-turn coil, but the one-turn coil does not function as a radiation element in itself. That is, the current generated in the one-turn coil is only an induced current generated by magnetic coupling to the antenna coil on the front surface, and this induced current is not large. For this reason, a magnetic field generated by the induced current is also not large, and it is difficult to use the one-turn coil as a radiation element.
Further, in a case in which some conductor pattern is provided on the back surface of the base material, as in the antenna device of Japanese Unexamined Patent Application Publication No. 11-3411, when a power feed circuit, such as an RFIC, is coupled to an inner peripheral end and an outer peripheral end of the rectangular spiral conductor pattern provided on the base material, a line for connecting the inner peripheral end and the power feed circuit needs to be formed on the surface where the rectangular spiral conductor pattern is provided and in a state insulated from the rectangular spiral conductor pattern. For this reason, the structure and formation method of the conductor pattern are complicated. Further, the line for connecting the inner peripheral end of the rectangular spiral conductor pattern and the power feed circuit, for example, a jumper line, causes deterioration of antenna characteristics.